This antenna relates generally to antennas for receiving or transmitting radio frequency energy and more particularly to broadband antennas which fit into relatively small volumes.
Antennas are widely used in many types of systems. The structure of the antenna affects its operating characteristics. As a result, antennas take on a variety of shapes. The particular shape is selected to meet the requirements of any system.
For certain systems, it is desirable to construct a broadband, circularly-polarized antenna which transmits or receives a beam of radio frequency energy over a wide angular range (i.e., a broad-beam antenna). The antenna must be relatively small and have a low manufacturing cost. Further, when many antennas are made, each should have substantially the same performance characteristics.
One type of broadband, circularly-polarized, broadbeam antenna is the spiral antenna. A spiral antenna contains two conductive traces formed into two interlocking spirals on a surface. The surface could be flat or cone shaped.
A cone shaped antenna provides greater directivity in the direction of the apex of the cone. However, cone shaped anennas are generally more difficult and costly to manufacture than flat spirals.
Known modifications to the basic spiral structure increase the bandwidth of the antenna. In one modification, the cavity behind a flat spiral is a constant depth and is filled with a material which absorbs RF energy. This structure increases the bandwidth but reduces the gain of the antenna. However, for small antennas (diameters less than one-half wavelength) with no absorber in the cavity, the gain is not reduced. Nevertheless, this technique provides only about a 2:1 bandwidth and does not provide bandwidths of 3:1 needed for some applications.
In another modification, the cavity is made cone shape so that it is deeper behind the center of the spiral. Such an antenna can operate over a 3:1 frequency range and has better gain, but at the upper frequencies in the range has a narrower coverage angle.
An additional problem exists with spiral antennas. The ends of the sprials must be coated with some form of material which absorbs RF energy to eliminate reflections from the ends of the spirals. When many antennas are produced, precise manufacturing techniques must be used to ensure that the absorber has the same thickness and dimensions on all such antennas. The precise techniques are costly to employ but if not followed, will result in variations in performance from antenna to antenna.